It was a day of jarring juxtapositions: The president of the United States was briefed about Dzhokhar Tsarnaev, the 19-year-old suspect in the Marathon bombing.

Then there was that other briefing — more intimate, more bizarre — that took place among the teenagers who knew him. His inner circle of friends, whose Twitter accounts are full of comments about smoking weed, being bored with classes, and partying suddenly turned to shock at the allegations against him.

“Regardless what happens that’s still my (expletive) . . . just want some answers,” wrote one of Tsarnaev’s friends, who goes by the Twitter handle TheZooKid and was using a racial slur as a term of affection. A few hours later, he wrote: “Don’t let them take you alive, bro.”

“Is this real life?” wrote NbeFly. “ . . . The president is involved like they refer to him as a terrorist. I can’t deal.”

There was another odd juxtaposition: A suspect who a day earlier seemed unknowable — who “must have been a sociopath,” as one TV talking head on the put it — came to seem recognizably human when described by his own friends, or when describing himself.

“I really don’t like it when I have one ear pressed against the pillow and I start to hear my heart beat,” Tsarnaev wrote on his Twitter feed weeks ago. “Who can sleep with all that noise?”

Strikingly, his postings showed the hallmark banality and absurdity of the average American teen. From the handle J—tsar, he tweeted about the TV shows “Game of Thrones” and “Breaking Bad.” He tweeted about smoking marijuana: “You don’t care if I smoke, right? Man, I wouldn’t care if you shot yourself in the head. Friendship.”

Four days before the bombing, he wrote: “Dreams really do come true. Last night I dreamt I was eating a cheeseburger and in the afternoon today, guess what I was eating.”

Then, in the midst of those tweets are ones that now seem macabre: “If you have the knowledge and the inspiration all that’s left is to take action,” he wrote earlier this month.

It’s hard — perhaps impossible — to glean a person’s mindset from 140-character Internet postings. Especially from teenagers, whose comments, like the revving of a motorcycle, are designed to show how tough they are. But still, we can see something in Tsarnaev’s tweets on Tuesday. As the city fell into mourning, they grew defiant.

Someone posted that iconic photograph of a man crouching over an injured woman at the Marathon finish line; underneath was a claim that the man was her boyfriend, planning to propose to her, but now she is dead. Tsarnaev was unmoved. “Fake story,” he tweeted back.

Later on Tuesday, he wrote: “So then I says to him, I says, relax bro my beard is not loaded” — an apparent reference to the stereotype of bearded Muslim men as terrorists. It feels bizarre and ironic now, given the circumstances. Is it even worth our time to dissect a comment like that? And what to make of what he wrote on Wednesday? Before the FBI released video footage of the suspects, he tweeted, seemingly out of the blue, “I’m a stress free kind of guy.”

He was probably anything but. Then he seemed to get more introspective. Did he understand that the world was closing in on him — that he would soon be identified as a criminal suspect?

His last tweet came on Wednesday, at 12:30 p.m.: a message from Mufti Ismail Menk, a Saudi scholar who appears to be based in Zimbabwe. “Attitude can take away your beauty no matter how good looking you are or it could enhance your beauty, making you adorable.”

Since then, silence.

Except that his Twitter page is busier than it has ever been. First, there were all the friends expressing shock and disbelief in whatever teen language they speak.

Then tens of thousands of strangers who signed up to “follow” him. Like me, they are looking for anything that can explain what might have happened; any insight into how this world of teens in saggy jeans collided with our world, and ended up in the president’s national security briefing.

We roam his Twitter feed like ghosts in a dead man’s brain; like beachcombers looking for precious metal, searching for something — anything — that can give us a reason for why a kid so harmlessly familiar may have hurt us so very much.